In internal combustion engine the masses of fuel that are injected per working cycle into the individual cylinders vary significantly because of manufacturing tolerances of a fuel injection system and by the occurrence of ageing of components of the fuel injection system. However, differences in the masses of the injected fuel result in torque differences between the individual cylinders that have an adverse effect on the smooth running of the internal combustion engine. Modern internal combustion engines, especially diesel engines, are therefore fitted with at least one so-called cylinder pressure sensor, which detects the time profile of the pressure in the interior of a cylinder. The torque provided by the cylinder involved can be estimated from the pressure profile and especially from the level of the pressure during the so-called working stroke in which the fuel combustion takes place. Based on a knowledge of such torque differences, balancing of the cylinders, i.e. equal torque contributions by all cylinders, can be achieved by means of an adjusted cylinder-specific fuel injection.
However, the output signal of a cylinder pressure sensor can be incorrect for many reasons. If such errors are not detected, this typically results in an incorrect cylinder-specific adjustment of the fuel injection. The smooth running of the internal combustion engine may not only not be improved but may even be significantly worsened.
A method for so-called cylinder balancing in relation to the injected masses of injected fuel in the different cylinders of an internal combustion engine is known From DE 197 20 009 A1. With this method the revolution rate or the rate of rotation during expansion and the revolution rate or the rate of rotation during compression is calculated for each cylinder. The difference in revolution rate between expansion and compression is filtered by means of a smoothing average value generation. Based on said filtered difference in revolution rates, an individual correction for the mass of fuel is calculated for each individual cylinder and said individual correction is taken into account during the calculation of the entire mass of fuel to be injected. The smooth running of the internal combustion engine can thus be improved by means of a mathematically relatively complex algorithm.
A method for compensating a systematic error in injection processes for an internal combustion engine is known from DE 197 00 711 A1. With this method, a correction value for the injection timing is used depending on the rough running.
A method and a system for cylinder balancing in reciprocating piston engines by compensating the harmonic components of the revolution rate of the crankshaft are known from DE 10 2005 047 829 B3. With this method a time interval of at least one revolution of the camshaft or two revolutions of the crankshaft is considered and within said time window a revolution rate signal of the crankshaft is subjected to a Fourier analysis.
The most frequent and fundamental cause of rough running of an internal combustion engine is, however, as explained above, a variation of the injected masses of fuel in the different cylinders. Assuming complete fuel combustion, different fuel-injected masses nevertheless result in different amounts of energy being released by fuel combustion in the working stroke of a cylinder of a four-stroke internal combustion engine.